feat: implement multi-factor authentication (MFA) setup and verification flow

- Add SecurityPage component for managing MFA settings, including enrollment and verification.
- Create MfaLoginGate to handle MFA verification state during login.
- Develop MfaVerificationPopup for user input of verification codes.
- Implement VerifyMfaPage for the MFA verification process after login.
- Introduce reusable VerificationCodeInput component for entering verification codes.
- Integrate Supabase MFA API for managing factors and verification.
- Add loading states and error handling for a better user experience.

backend/schema.sql

@@ -19,6 +19,7 @@ create table if not exists public.user_profiles (
   title_model text,
   tabular_model text not null default 'gemini-3-flash-preview',
   quote_model text,
+  mfa_on_login boolean not null default false,
   created_at timestamptz not null default now(),
   updated_at timestamptz not null default now()
 );